1. Technical Field
This invention relates generally to a seal and seal assembly for sealing against a rotatable shaft. In particular, the invention is concerned with a labyrinth type seal such as may be used to seal against the leakage of gas between high and low pressure areas in a compressible fluid turbo machine.
2. Background Information
In a compressible fluid turbo machine such as a high speed centrifugal compressor, it is important to the efficiency of the machine that leakage of compressed gas between high and low pressure areas be kept as low as possible. This is particularly true along the rotatable shaft carrying the compressor impeller where the shaft extends through a support in the housing from an area of high pressure to an area of low pressure. One prior arrangement has been to utilize a fixed labyrinth seal assembly to seal between the housing and the shaft in the areas between high and low pressure. With such an arrangement, an annular seal body is mounted within an annular cavity in the housing. The impeller shaft extends through the seal body with a labyrinth portion of the body acting between the shaft and a sealing surface on the shaft to keep high pressure gas from leaking along the shaft into the low pressure area. As operating conditions of the compressor create higher pressure differentials between high and low pressure areas and the rotational speed of the shaft increases, closer tolerances are required between the labyrinth and shaft seal surfaces in order to keep leakage within acceptable limits. However, a problem is encountered as closer tolerances are required in that vibration of the shaft as it rotates may exceed tolerance clearances, causing the shaft to wear against the seal and premature failure of the seal.